1. Conventional Arts
In recent years, utilization of a wireless communication system using a 2.4 GHz band that is a bandwidth available without a license is expanding.
The 2.4 GHz band is referred to an ISM (Industry-Science-Medical) band, and is used for various kinds of industrial equipment and medical instruments other than systems compliant with standards of wireless systems, such as a wireless LAN of the IEEE 802.11 system, Bluetooth (registered trademark), and Zigbee (registered trademark).
Other than these, there is a CBTC (Communication Based Train Control) system that is a railroad signaling system, as one of the wireless communication systems using the 2.4 GHz band.
The railroad signaling system is designed on fail-safe basis, wherein the railroad signaling system controls so as to be on a safe side even if any failure occurs to the system, and is a key system for assuring a safe operation of trains.
Since the railroad signaling system has an important function in railroad transportation, a transport failure due to stoppage of the system has an extraordinary effect on the society. Therefore, there is a need for a wireless communication system that guarantees high reliability (high operating rate).
However, in order to achieve reliable communication in the wireless system using the 2.4 GHz band, particularly the following four interferences need to be taken into consideration.
The first one is the interference from a wireless LAN (WLAN) or the like, the second one is the interference to other system, such as a WLAN, the third one is the interference due to a plurality of base stations (AP's) using the same frequency channel (referred to as intra-system interference), and the fourth one is the interference occurring between a plurality of adjacent CBTC systems (referred to as inter-system interference) for example when there are a plurality of tracks (railways) running in parallel.
With regard to the first interference, taking into consideration recent proliferation of the wireless LAN, a number of interferences may occur and thus particularly a countermeasure needs to be taken.
From the above, it is not easy to achieve reliable communication in the CBTC system using the 2.4 GHz band in which there are a number of interferences.
2. Related Arts
The related arts include JP-A-2009-225135, “METHOD FOR SELECTING FREQUENCY CHANNEL IN TRAIN RADIO COMMUNICATION SYSTEM AND ON-BOARD RADIO TERMINAL” (Patent Literature 1), Hitachi, Ltd., and JP-A-2009-171078, “RADIO COMMUNICATION SYSTEM, BASE STATION, AND FREQUENCY CHANNEL SELECTION METHOD” (Patent Literature 2), Hitachi, Ltd.
Patent Literature 1 describes that communication with a base station (AP: Access Point) installed along a track of a train is performed using mobile stations (STA: Station) installed in front and at the back of a train set.
STA performs channel search based on a pilot signal from the AP to hand over, thereby realizing seamless communication.
Transmission between AP and STA shall be repeated twice (referred to as “continuous transmission”), thereby achieving time diversity. Moreover, a different frequency channel is used for each AP and furthermore frequency hopping is performed wherein a frequency channel is changed every time transmission is performed, thereby achieved frequency diversity as well.
Furthermore, STA of an identical train set shall send identical train control information, thereby achieving space diversity. With the time diversity, frequency diversity, and space diversity described above, reliable communication is realized.
Patent Literature 2 proposes a frequency hopping method based on Patent Literature 1.
In Patent Literature 2, first and second hopping tables are used for frequency hopping. Basically, the frequency hopping is performed according to the first hopping table, but when the communication quality of a frequency channel corresponding to a phase indicated from an AP master (APM), which is a centralized control apparatus of an AP, is poor, this AP determines a frequency channel to use, according to the second hopping table. Thus, the utilization of a frequency channel having a number of interferences and poor communication quality is avoided.
However, during the next transmission, the first hopping table shall be used again, and whether or not the second hopping table is used is determined again based on the communication quality.